Method and apparatus for measuring tooth tightness

ABSTRACT

According to the present invention there is provided an instrument for measuring the tightness between a first and a second contiguous bodies, the first body being movable with relation to the second body, comprising: (a) a substantially incompressible insertion element of predetermined thickness for insertion between the first and second bodies; (b) a holder for holding the insertion element to enable the insertion element to be forcibly inserted between the first and second bodies, thereby displacing the first body with relation to the second body, thereby providing a predetermined forced distance between the first and second bodies; and (c) a sensor for sensing the insertion force applied when inserting the insertion element between the first and second bodies along a predetermined path of insertion. Further according to the present invention there is provided a method of measuring the tightness between a first and a second contiguous bodies, the first body being movable with relation to the second body, comprising the steps of: (a) forcibly inserting a substantially incompressible insertion element of predetermined thickness between the first and second bodies; and (b) simultaneously measuring the insertion force required for displacing the first body with relation to the second body so as to provide a predetermined forced distance between the first and second bodies, wherein said measuring is performed along a predetermined path of insertion by means of a sensing element.

The present invention relates to a method and also to an apparatus formeasuring the tightness between two contiguous bodies. The invention isparticularly useful for measuring the tightness of contiguous dentalbodies, such as dental teeth, and are therefore described below withrespect to this application, it being understood that the method andinstrument according to the present invention is not limited to dentalapplications.

In the fields of orthodontics, periodontics and oral rehabilitation,there are many instances where it is desirable to measure the tightness(including looseness or spacing) between two contiguous teeth. Forexample at the end of an orthodontic treatment, removable retentiveappliances are placed in the mouth to maintain the accomplished result.The wearing time of the retentive appliances is successively reduceduntil a steady state of the oral tissue is achieved. However, the extentof this recovery process is individual, and therefore if the recoveryprocess is not monitored, there is a danger of a relapse to or towardsthe original condition which was to be corrected. A similar situationarises when a crown installed over a metal implant embedded in thealveolar bone, since an excessively tight contact point between a crownlinked to the implant and an adjacent tooth can lead to a collapse ofthe bony bearing system.

U.S. Pat. No. 4,571,181, and 4,664,627 describe mechanical type dentalthickness gauges for measuring the spacing between teeth, but suchmechanical gauges are relatively inaccurate and generally unsuitable foruse in continuously monitoring a recovery process, such as describedabove. A publication by Thomas E. Southard et al., entitled "AnteriorComponent of Occlusal Force, Part 1--Measurement and Distribution, Am.J. Orthod. Dentofac. Orthop., December 1989, pp. 439-500, describes, forthis purpose, the use of a stainless steel strip slipped between the twoteeth and withdrawn by the use of a digital tension transducer tomeasure the frictional force resisting the withdrawal of the strip.

An object of the present invention is to provide another method andinstrument for measuring the tightness of two contiguous bodies,particularly dental teeth.

According to the present invention, there is provided a method ofmeasuring the tightness of two contiguous bodies by forcibly insertingan insertion element of predetermined thickness between the two bodies;and simultaneously measuring the insertion force, preferably includingthe peak of the insertion force. As indicated earlier, the novel methodis particularly useful for measuring the tightness of dental teeth,e.g., to permit continuous monitoring of the recovery process of anorthodontic treatment. The invention thus provides a diagnostic tool topredict hazardous side effects such as tissue relapse.

According to further features in the described preferred embodiment, theinsertion element is forcibly inserted between the two teeth byattaching the opposite ends of the insertion element to a holder, andmanually forcing the holder, with the insertion element attachedthereto, inwardly between the two teeth towards the gingiva. Inaddition, the insertion element is a strip of predetermined thickness,preferably of stainless steel.

The peak of the insertion force is preferably measured by one or morestrain gauges. It is possible, however, to measure the peak insertionforce by measuring displacement by an optical system, a piezoelectricdevice, capacitance, inductance, potentiometer, or by pneumatic device,a manometer pressure device, or by the Hall effect.

It will thus be seen that the novel method of the present inventiondistinguishes over that in the Southard et al. publication in a numberof important respects. Thus, the technique proposed by Southard et al.(Southard, T. E., Behrents, R. G., Tolley, E. A., The Anterior Componentof Occlusal Force, Part 2, Relationship with Dental Malalignment, Am. J.Orthod. Dentofac. Orthop. 97; 41-4, 1990), is based on the insertion ofa stainless steel strip between two teeth and the subsequent drawing ofthe strip laterally between the two teeth. It is during this lateraldrawing that tension measurements are made. The measurements are thusprimarily a combination of the tightness of adjacent teeth and thefrictional forces resisting the lateral withdrawal of the steel strip.Therefore, the friction between the teeth, which is often influenced byextraneous factors, such as the presence of food particles, plaque,calculus and saliva, largely influences the tension reading and tends tomask the true tightness, thus making it impossible to distinguishbetween the true effect (pure tightness) and the hiss (friction).

By contrast, the present invention, involves the forcible insertion ofthe insertion elements between two teeth and the simultaneous measuringof the insertion force. The measurements are thus related more directlyto the prying or wedging apart of the two teeth and are thus moreclosely related solely to the tightness of the two teeth.

Furthermore, in the Southard method, the teeth are first moved apart andthen try to approach each other and thereby applying force on theinsertion element. However, the very act of inserting an insertionelement between a pair of teeth tends to temporarily loosen theperiodontal fibers which connect the teeth to the supporting bone.Because the periodontal fibers have viscoelastic properties they willresort to their original shape after an undetermined time. Thus, duringthe lateral drawing of the strip in the Southard method the teeth arealready considerably looser than they were prior to the insertion of thestrip, rendering the measurements less accurate. By contrast, under thepresent invention, the measurements are conducted during the initialinsertion of the insertion element simultaneously with the displacementof the teeth.

The invention also provides an instrument for measuring the tightness oftwo contiguous bodies, particularly dental teeth, in accordance with theabove method.

Further feature and advantages of the invention will be apparent fromthe description below.

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 illustrates one form of measuring instrument constructed inaccordance with the present invention;

FIG. 2 illustrates a set of the insertion elements or strips ofdifferent thicknesses which may be used in the instrument of FIG. 1;

FIG. 3 illustrates typical results obtained when using three of the fourdifferent insertion strips illustrated in FIG. 2 between the samecontiguous teeth;

FIG. 4 illustrates results produced when using a strip of singlethickness for measuring teeth tightness with respect to several pairs ofteeth during different phases of orthodontic treatment;

FIG. 5 is a typical strain curve; and

FIG. 6 shows an embodiment of a device according to the presentinvention.

With reference first to FIG. 1, there is illustrated one form ofinstrument for measuring the tightness of two contiguous bodies,particularly dental teeth, in accordance with the present invention.

The instrument illustrated in FIG. 1 includes a holder, generallydesignated 2, for holding an insertion strip 4 of predeterminedthickness, such as to permit the strip to be forcibly inserted betweenthe two dental teeth while a measurement is made of the force during theinsertion or penetration between the two teeth. Preferably, themeasurement includes a determination of the peak force during theinsertion. An analysis of the force function, preferably including thepeak of the insertion force, together with the thickness of theinsertion strip 4, provide an objective indication of the tightness ofthe two teeth such that the tightness can be accurately measured, notonly before and during the orthodontic treatment, but also after thetreatment to monitor the recovery process in order to determine thestability of the results and to predict any relapse tendency in order totake whatever corrective action may be necessary to preserve thetreatment results. A typical insertion force function is represented inFIG. 5. Here, the insertion force increases as the insertion element ispushed downward or upward between two adjoining teeth (or other bodies).The force peaks and then drops off again. A slight increase is typicallyobserved when the insertion member impacts the gums. Negative tightnessunits are recorded when the strip is pulled out of the contact point ofadjacent teeth.

Holder 2 includes a manually-graspable handle 10 mounting a U-shapedframe member 12 to which the opposite ends of the insertion strip 4 areattached. The juncture 14 between the handle 10 and the U-shaped frame12 includes one or more strain gauges 16 which measures the strain,which is related to the insertion force, at the time strip 4 is forciblyinserted between the two teeth in order to measure the tightnessthereof.

The U-shaped frame 12 is constituted of a rod bent into a U-shape andterminating in end extensions 18, 20 which are received within openings22, 24 formed in the opposite ends of the insertion strip 4. Thus, endextensions 18, 20 of the U-shaped frame serve as attaching elementsreceivable within openings 22, 24 of the strip 4 for firmly attachingthe strip to the U-shaped frame 12. To facilitate attaching strip 4, theframe is formed with a loop 26 providing flexibility to the end 24 ofthe frame, enabling that end to be engaged in the hole 24 of the strip4.

Strain gauge 16, which senses the insertion force when inserting strip 4between two teeth, is connected to a measuring circuit 30 for measuringthe force function, preferably including at least the peak insertionforce, and also to a data acquisition system 32 for processing,recording and/or displaying the measured insertion force. Preferably,the measuring circuit 30 (which may be a Wheatstone Bridge) and the dataacquisition system 32 are housed within the handle 10 for ease of use ofthe device. The handle 10 (FIG. 6) preferably also includes a powersupply 31 and preferably also a suitable display 33 for displaying theresults to the user.

The one or more strain gauges 16 may be located outside of the handle 10(as in FIG. 1) or may alternatively be located within the housing 10 (asin FIG. 6), the latter configuration offering added protection andreliability to the strain gauges 16 but requiring that the U-shapedframe member 12 and the insertion strip 4 be handled as a single item,i.e., that the combination of the U-shaped frame 12 and the insertionstrip 4 be changed when desired, rather than changing merely theinsertion strip 4 as in the embodiment of FIG. 1.

As indicated earlier, the insertion force depends to a great extent onthe thickness of the insertion strip 4. For this reason, the instrumentwould be provided with a plurality of such insertion strips of differentthicknesses. This is illustrated in FIG. 2, wherein, for purposes ofexample, strip 4a is of 0.05 mm, strip 4b is of 0.10 mm, strip 4c is of0.15 mm and strip 4d is of 0.20 mm.

The manner of using the instrument illustrated in FIGS. 1 and 2 will beapparent from the above description.

Thus, when instrument is to be used for measuring teeth tightness, astrip 4 of suitable thickness is selected according to the particularcase and is attached to the ends 18, 20 of the U-shaped frame 12. Handle10 of the instrument is manually grasped with the strip 4 aligned in thespace or contact point between the two teeth, and is pressed firmlytowards the subject's gingiva to cause the strip 4 to be inserted orpenetrate between the two teeth. The force required to do this is sensedby the strain gauge 16, which outputs an electrical signal to themeasuring circuit 30. Circuit 30 measures the insertion force,preferably including the peak insertion force, and feeds thisinformation to the data acquisition circuit 32, which records, displaysand/or otherwise processes this information.

FIG. 3 illustrates the results of using the instrument illustrated inFIGS. 1 and 2 during a typical orthodontic treatment involving an activephase Pa during which a fixed orthodontic appliance was applied to thesubjects teeth, a deboning phase Pd when the fixed orthodontic appliancewas removed, and a retention phase Pr during which a removable retentionappliance was applied. In the example illustrated in FIG. 3, theinsertion strip 4 was applied between the same two teeth, i.e., at thesame single contact point indicated by the arrow 40 in the upper righthand corner of FIG. 3. Curve A in FIG. 3 indicates the microstrain (inarbitrary tightness units) over the indicated period of time when theinsertion strip 4 was strip 4a in Fig. 22, of 0.05 mm; curve B indicatesthe results when the insertion strip was 4b in FIG. 2, namely 0.10 mm;and curve C illustrates the results when the strip 4c, namely 0.15 mmwas used.

FIG. 4 illustrates further typical results when a strip thickness of0.15 mm was used at three contact points: teeth 2, 3; 3, 5; and 5, 6 ofthe fourth quadrant (right mandibular dentition). Thus, curve A showsthe results when the measurement was made between teeth 2 and 3; curve Bshows the results when the measurement was made between teeth 3 and 5(teeth 4 having been removed); and curve C shows the results when themeasurement was made with respect to teeth 5 and 6. The drop in themeasurement indicated by the arrow (FIG. 4, contact point 45-46) is dueto the removal of a band which was wrapped around the tooth 46. Due tothe thickness of the band a gap was produced between the adjacent teethwhich caused the sharp drop in the tightness.

While the invention has been described with respect to one preferredembodiment, it will be appreciated that this is set forth merely forpurposes of example. Thus, other force measuring system could be used,for example by an optical system for measuring displacement, apiezoelectric system for measuring pressure changes, a capacitance,inductance or resistor-potentiometer system for measuring displacement.The measurement could also be pneumatically, by a manometric pressuredevice, or by the magnetic Hall effect.

In addition, other means can be provided for attaching the insertionstrips 4 to the instrument, e.g., arrangements for attaching blades tohacksaws or jigsaws. Also, a single holder can hold a plurality of suchinsertion strips each made selectively operable. Further, the inventioncould be advantageously used for measuring tightness between otherbodies.

Many other variations, modifications and applications of the inventionwill be apparent.

What is claimed is:
 1. A method of measuring the tightness between afirst and a second contiguous bodies, the first body being movable withrelation to the second body, comprising the steps of:(a) forciblyinserting a substantially incompressible insertion element ofpredetermined thickness between the first and second bodies; and (b)simultaneously measuring the insertion force required for displacing thefirst body with relation to the second body so as to provide apredetermined forced distance between the first and second bodies, saidpredetermined forced distance being substantially equal to saidpredetermined thickness of said insertion element, wherein saidmeasuring is performed along a predetermined path of insertion by meansof a sensing element.
 2. A method according to claim 1, wherein saidmeasuring the insertion force includes measuring the peak of theinsertion force.
 3. The method according to claim 1, wherein said firstand second contiguous bodies are first and second dental teeth.
 4. Themethod according to claim 3, wherein said insertion element is forciblyinserted between the first and second teeth by attaching the insertionelement to a holder, and manually forcing the holder, with the insertionelement attached thereto, inwardly between the first and second teethtowards the gingiva.
 5. The method according to claim 1, wherein saidinsertion element is a strip of predetermined thickness.
 6. The methodaccording to claim 5, wherein said strip is of stainless steel.
 7. Themethod according to claim 1, wherein at least one of the first andsecond bodies is flexible.
 8. The method according to claim 1, whereinat least one of the first and second bodies is connected to a flexiblesubstrate.
 9. An instrument for measuring the tightness between a firstand a second contiguous bodies, the first body being movable withrelation to the second body upon insertion of said instrumenttherebetween, comprising:(a) a substantially compressible insertionelement of predetermined thickness for insertion between the first andsecond bodies; (b) a holder for holding said insertion element to enablethe insertion element to be forcibly inserted between the first andsecond bodies, thereby displacing the first body with relation to thesecond body, thereby providing a predetermined forced distance betweenthe first and second bodies, said predetermined forced distance beingsubstantially equal to said predetermined thickness of said insertionelement, and (c) a sensor for sensing the insertion force applied wheninserting said insertion element between the first and second bodiesalong a predetermined path of insertion.
 10. An instrument according toclaim 9, wherein said sensor senses at least the peak of the forceapplied when inserting said insertion element between first and secondbodies along a predetermined path of insertion.
 11. The instrumentaccording to claim 9, wherein said holder comprises a manually graspablehandle, and a U-shaped frame, said frame carrying a first attachingelement at said first end and a second attaching element at said secondend, said first attaching element for attachment to said first end ofsaid insertion element, and said second attaching element for attachmentto said second end of said insertion element.
 12. The instrumentaccording to claim 11, wherein said insertion element is a strip ofpredetermined thickness, said strip having first and second ends. 13.The instrument according to claim 12, wherein said strip is formed withopening at its first and second ends for attaching the strip to saidattaching elements of the U-shape frame.
 14. The instrument according toclaim 13, wherein one of said attaching elements is movable with respectto the other parallel to the longitudinal axis of the insertion strip tofacilitate attachment of the first and second ends of the insertionstrip.
 15. The instrument according to claim 14, wherein said frameincludes a rod bent into a U-shape and formed with a loop to permitmovement of said first attaching element at said first end of said framewith respect to said second attaching element at said second end of saidframe to facilitate attaching of said insertion strip thereto.
 16. Theinstrument according to claim 10, further comprising a plurality of saidinsertion strips of different predetermined thickness selectivelyattachable to said first and second ends of said U-shaped frame.
 17. Theinstrument according to claim 12, wherein said sensor is carried at thejuncture of said handle and said U-shaped frame.
 18. The instrumentaccording to claim 12, wherein said sensor is a strain gauge.
 19. Theinstrument according to claim 11, further including a measuring circuitfor measuring the force sensed by said sensor and a data processor forprocessing and/or recording and/or displaying said measurement.
 20. Theinstrument according to claim 19, wherein said measuring circuit andsaid data processor are located in said handle.
 21. The instrumentaccording to claims 9, wherein at least one of the first and secondbodies is flexible.
 22. The instrument according to claim 9, wherein atleast one of the first and second bodies is connected to a flexiblesubstrate.